Trying to calculate the difference in dates using =IF Formula. I am new to Excel

Hi, totally new to excel, trying to apply formulas from a previous excel sheet to another.

Hello, I'm very new excel and not very good at it. I work a corporate job and I'm trying to calculate the days between two dates(so if M is 1/1/2026 and O is 1/2/2026 i need the difference in column N). I have a formula from a previous spreadsheet and I keep trying to apply it to the new spreadsheet but it keeps coming back as #VALUE!.

I also put the formula in Copilot and I can't forgive the life of me figure out the formula or why its not working. I also need for the whole column with different dates.

the formula from a previous spreadsheet is =IF(ISBLANK(E139)" "IF(ISBLANK(T39),TODAY()-E139T139-E139))

Hopefully this makes sense. I'm honestly not great with Excel and trying my best. If someone could help that'd be great. Thankyou so much!

EDIT: You all have been wonderful! TY!!!